Exhaust streams generated by the combustion of fossil fuels contain different types of pollutants, such as carbon monoxide (CO), oxides of sulfur (SOx; x=2 or 3), unburned hydrocarbons (HCs), volatile organic compounds (VOCs) and nitrogen oxides (NOx). While there are different catalytic devices and methods to treat exhaust streams, it is difficult to remove such pollutants as NOx and SOx from the exhaust stream.
The rate of conversion of the pollutants often depends on the exhaust gas temperatures. For example, automobile catalytic converters may need to operate at an elevated temperature of about 300° C. or higher. However, there is an initial time period between when the exhaust emissions begin (i.e., “cold start”) and the time when the catalytic converter heats up. This time is often called the “light-off time.” The “light-off temperature” is the catalyst temperature at which fifty percent (50%) of the emissions from the engine convert as they pass through the catalyst; it is represented as T50.
While the exhaust gases may heat the catalytic converter and such heating helps in bringing the catalyst to the light-off temperature, the initial exhaust gases generally pass through the catalytic converter relatively unchanged until the light-off temperature. Also, it has been found that the composition of the exhaust gas itself changes as the temperature increases from a cold start temperature to the operating temperature.
The earliest automotive catalysts were developed in the mid '70s to facilitate quantitative oxidation of carbon monoxide and unburned hydrocarbons to CO2 and H2O. Almost all the earlier automotive catalysts comprised of an inert ceramic support loaded with noble metal, as precious metals are known to be excellent oxidation catalysts. However, the catalysts come at a premium price, which has increased to an unprecedented level in the past 5 years. In particular, the amount of such precious metals as Rh, Pt and Pd used for automotive catalysts has increased as a whole over the last decade. Among these precious metals, the demand for Pd rapidly grew since there was an issue related to the need of reducing the cold start hydrocarbon (HC) and carbon monoxide (CO) emissions.
This increasing demand has led to the search and use of alternative formulations that have less expensive and more readily available base metal oxides such as those of Co, Ni, Cu and even Cr. However, due to their hydrothermal stability and propensity to poisoning by the presence of sulfur inherent in the fuel, these base metals do not qualify to meet the stringent specifications that are required for automotive catalysts. Hence, precious metal-catalyzed systems became the only viable automotive catalysts.
It is therefore desirable to have a catalyst composition that is useful both before and after the light off temperature is reached.
It is also desirable to have a catalyst composition that is effective for emission reductions across a range of temperatures and operating conditions that differ from those currently available.
It is also desirable to have a catalyst composition that is effective in reducing such different types of pollutants as CO, SO2, and NOx, in addition to removing HCs and VOCs.